In many optical storage systems that employ magnetic materials, data is represented by magnetized domains. The direction of magnetization of the domains is generally perpendicular to the plane of the medium which is, in turn, a thin magnetic film. Data is read from the medium using a magneto-optical effect, i.e., a shift of polarization direction of a light beam when it encounters the magnetized medium. The polarization shift is a function of the direction of magnetization of the domain. Thus, by sensing reflected polarized light, the identity of a bit can be determined.
The magnetic film is initialized during manufacture by applying a field to magnetize it in one direction. Bits are written by heating selected areas of the film above its Curie point T.sub.c and then cooling the areas in the presence of an orienting magnetic field. The coercive force within the film (the field that keeps the domains in place and stable in size), decreases with increasing temperature, as T.sub.c is approached. This writing technique is known as thermomagnetic writing.
The magnitude of the orienting magnetic field is adjusted so that it can only overcome the film's coercive forces when the material is heated to approximately T.sub.c. Selective heating is achieved through the use of a laser beam, electron beam or other directed energy beam.
One problem with ferromagnetic writing of magneto/optical memory cells is the time required to heat a bit position to T.sub.c. Generally, a ferromagnetic film is chosen with a T.sub.c as close to room temperature as possible to speed up the write process and to allow the use of a lower power addressing beam. This allows the writing speed to be increased, decreases the required beam power, and avoids heating of the magnetic material to a level where the material is irreversibly damaged and loses its magnetic properties. The drawback of using a material with a T.sub.c close to room temperature is that the magneto-optic rotation of the domain is reduced and, as a result, the amount of induced polarization shift during the read process is similarly reduced.
The prior art is replete with references relating to various magneto-optical data storage systems.
In U.S. Pat. No. 3,949,387 to Chaudhari et al., a beam addressable system is described using a ferromagnetic write procedure. A variety of ferromagnetic amorphous magnetic compositions are described that enable high levels of uniaxial anisotropy to be obtained. In U.S. Pat. No. 4,649,519 to Sun et al., a bilevel storage media is described wherein a ferrimagnetic film is used to bias a ferromagnetic layer during a write action. The ferromagnetic layer is heated above its Curie temperature and when it cools, the biasing layer applies a fringe field which aids in the reorientation during cooling.
In U.S. Pat. No. 4,731,754 to Odgen, a Polyvinyl fluoride (PVF) ferroelectric with a T.sub.c lower than its melting point is employed as the storage media. A laser beam heats the PVF film and a bias layer causes the film to selectively orient in the heated areas. In U.S. Pat. No. 4,794,560 to Bell et al., a three layer magneto-optic recording media is shown wherein an intermediate thermal isolation layer is positioned between two magnetic layers. This structure enables one layer to be utilized as an orienting layer while the other is selectively heated. The fringe field from the orienting layer then causes a preferred direction of reorientation, upon cooling.
In Japanese patent 63-316,344, a three layer magneto-optic recording system is shown, with a first layer comprised of a soft magnetic material with a high T.sub.c ; a second layer including a vertically magnetized thin film with a lower T.sub.c temperature; and a third vertically magnetized recording layer. The first and second layers are employed as aligning layers, in that, upon cooling one material reorients before the other. In Japanese patent 63-311645, a two layer magneto-optic recording system is shown, again using a bias medium for domain orientation.
While the prior art employs thermal means to modify a coercive field in a ferromagnetic memory film, other physical actions have been used to effect the coercive field, e.g., stress waves. Such a system is described in "Stress-operated Random Access, High-Speed Magnetic Memory" by Schroder, Journal of Applied Physics, Vol. 53, No. 3 March 1982, pages 2759-2761. The memory disclosed by Schroder includes a checkerboard grid of stress-wave transducers, each transducer coupled to two lines of conductors. The grid is perpendicular and a magnetic block is placed on the top of each transducer. A magnetic field is applied to the system of magnetic blocks and is somewhat smaller than their coercive field.
When a transducer in Schroder's system generates a stress within a magnetic block, the magnetic dipoles in the block align parallel to an applied field. If the induced stress is high enough, it enables a "writing" to occur.
Thin film memories have also been operated employing induced stresses and one such is shown in Japanese patent 62-249,408 wherein a ferromagnetic thin film is selectively stressed to provide magnetic anisotropy after the formation of the film. This patent's use of stress is not for the actual storage procedure but for the initial polarization of the memory material.
A further method for writing into a magneto-optical memory is described in "Magnetoptic Direct Overwrite Using a Resonant Bias Coil" by D. Rugar, IEEE Transactions on Magnetics, Vol. 24, No. 1, January 1988, pp. 666-669. A sinusoidal bias field is generated by an LC coil. Domains of either sign are then written by timing laser pulses with either positive or negative peaks of the bias field.
The following articles appear in the MRS BULLETIN, Vol. 15, No. 4, April 1990 and are useful in understanding the state-of-the-art relative to magneto-optical storage: Optical Storage Disk Technology, by Gambino, pages 20-22, "Materials Challenges and Integrated Optical Recording Heads", by Weller-Brophy et al., pages 25-30, and "Magneto-Optical Storage Materials", by Greidanus et al., pages 31-39.
Accordingly, it is an object of this invention to provide an improved magneto-optic thin film memory.
It is another object of this invention to provide an improved magneto-optic thin film memory with improved levels of signal output.
It is still another object of this invention to provide a higher speed magneto-optic thin film memory that employs ferromagnetic writing.